If you get javascript errors like

jQuery is Not Defined
or
$ is Not Defined

its almost always the case you are either not referencing the jquery javascript file or reference is wrong.

if you are not referencing the jquery file, you can download it from here http://docs.jquery.com/Downloading_jQuery

If you dont want to download you can always reference to the hosted jquery file. Links to some hosted jquery files can also be found here http://docs.jquery.com/Downloading_jQuery

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From Webcosmo Webmaster Forum http://www.webcosmoforums.com/javascript-ajax/25013-jquery-not-defined.html